The inflammatory response is an important element of a host's natural defense mechanism against pathogens. It also is involved in wound healing. Despite the beneficial role that the inflammatory response plays in host survival, excessive inflammation may have clinically adverse results in some medical conditions.
Leukocytes are a major cellular component of inflammatory and immune responses. This class of cells includes neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Neutrophils, which play a key role in the inflammatory response, are generally present within the body in a resting unstimulated state. Once stimulated, the neutrophils migrate to the site of injury and release toxic factors.
The migratory capability of a neutrophil is dependent on the ability of the neutrophil to alter its adhesive properties. In a resting unstimulated state a neutrophil is not adhesive and cannot migrate. Once the neutrophil has been stimulated, however, it becomes more adhesive and is capable of migrating. The increase in neutrophil adhesiveness causes the stimulated neutrophil to aggregate and to adhere to endothelium. Stimulation of the neutrophil also causes the neutrophil to undergo diapedesis, which involves the migration of the neutrophil between post-capillary endothelial cells into the tissues.
In the tissues, an activated neutrophil releases enzymes such as collagenase and elastase, among others. Neutrophil stimulation may also initiate a burst of oxygen consumption, with concomminant activation of the hexose-monophosphate shunt and activation of nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase. Activation of these systems results in the formation and release of factors such as hydrogen peroxide and hydroxyl radicals, which are toxic to microorganisms and tumor cells, and thereby facilitating the destruction of the injury causing agent.
Several studies have focused on analyzing the control and regulation of the adhesive properties of neutrophils. Much of this research has centered on adhesion receptors and also on metabolites of arachidonic acid such as C20 carbon fatty acid found in every cell membrane. Arachidonic acid metabolism occurs by different mechanisms in stimulated versus unstimulated neutrophils and results in the production of a different spectrum of metabolites in stimulated versus unstimulated neutrophils.
In stimulated neutrophils, the cytochrome P450 mixed function oxidase system appears to be more active. Moreover, during neutrophil stimulation, 5-lipoxygenase is translocated to the membrane compartment fraction, where it produces 5-hydroperoxyeicosatetraenoic acid (5-HPETE). 5-HPETE is then either metabolized to 5-hydroxyeicosatetraenoic acid (5-HETE) by peroxidase or dehydrated to form leukotriene A4. Leukotriene A4 is converted into leukotriene B4 which is a potent chemotactic agent and promoter of neutrophil adhesion.
In unstimulated neutrophils, the metabolism of arachidonic acid is markedly different than that in stimulated neutrophils. The metabolism of arachidonic acid in unstimulated neutrophils is sensitive to cytochrome P450 inhibitors but not to cyclooxygenase or lipoxygenase inhibitors. Hatzelmann and Ullrich characterized the metabolites produced in unstimulated neutrophils, reporting the finding that arachidonic acid is metabolized to 20-HETE and 15-HETE. Hatzelmann, Eur. J. Biochem. 173, 445-452 (1988). Another study, Kraemer et al., found that the arachidonic acid metabolic products formed in unstimulated neutrophils exhibited a potent anti-aggregatory activity toward human neutrophils, suggesting that the identified arachidonic acid metabolites may play some role in the regulation of neutrophil adhesion and aggregation properties. Kraemer et al., Am. J. Pathol. 128, 446-454 (1987).